Ink jet printers have become very popular due to their quiet and fast operation and their high print quality on plain paper. A variety of ink jet printing methods have been developed.
In one ink jet printing method, termed continuous jet printing, ink is delivered under pressure to nozzles in a print head to produce continuous jets of ink. Each jet is separated by vibration into a stream of droplets which are charged and electrostatically deflected, either to a printing medium or to a collection gutter for subsequent recirculation. U.S. Pat. No. 3,596,275 is illustrative of this method.
In another ink jet printing method, termed electrostatic pull printing, the ink in the printing nozzles is under zero pressure or low positive pressure and is electrostatically pulled into a stream of droplets. The droplets fly between two pairs of deflecting electrodes that are arranged to control the droplets' direction of flight and their deposition in desired positions on the printing medium. U.S. Pat. No. 3,060,429 is illustrative of this method.
A third class of methods, more popular than the foregoing, is known as drop-on-demand printing. In this technique, ink is held in the pen at below atmospheric pressure and is ejected by a drop generator, one drop at a time, on demand. Two principal ejection mechanisms are used: thermal bubble and piezoelectric pressure wave. In the thermal bubble systems, a thin film resistor in the drop generator is heated and causes sudden vaporization of a small portion of the ink. The rapidly expanding ink vapor displaces ink from the nozzle causing drop ejection. U.S. Pat. No. 4,490,728 is exemplary of such thermal bubble drop-on-demand systems.
In the piezoelectric pressure wave systems, a piezoelectric element is used to abruptly compress a volume of ink in the drop generator, thereby producing a pressure wave which causes ejection of a drop at the nozzle. U.S. Pat. No. 3,832,579 is exemplary of such piezoelectric pressure wave drop-on-demand systems.
The drop-on-demand techniques require that under quiescent conditions the pressure in the ink reservoir be below ambient so that ink is retained in the pen until it is to be ejected. The amount of this "underpressure" (or "partial vacuum") is critical. If the underpressure is too small, or if the reservoir pressure is positive, ink tends to escape through the drop generators. If the underpressure is too large, air may be sucked in through the drop generators under quiescent conditions. (Air is not normally sucked in through the drop generators because the drop generators comprise capillary tubes which are able to draw ink against the partial vacuum of the reservoir.)
The underpressure required in drop-on-demand systems can be obtained in a variety of ways. In one system, the underpressure is obtained gravitationally by lowering the ink reservoir so that the surface of the ink is slightly below the level of the nozzles. However, such positioning of the ink reservoir is not always easily achieved and places severe constraints on print head design. Exemplary of this gravitational underpressure technique is U.S. Pat. No. 3,452,361.
Alternative techniques for achieving the required underpressure are shown in U.S. Pat. No. 4,509,062 and in application Serial No. 07/115,0l3 filed Oct. 28, 1987, now 4,791,438, both assigned to the present assignee. In the former patent, the underpressure is achieved by using a bladder type ink reservoir which progressively collapses as ink is drawn therefrom. The restorative force of the flexible bladder keeps the pressure of the ink in the reservoir slightly below ambient. In the system disclosed in the latter patent application, the underpressure is achieved by using a capillary reservoir vent tube that is immersed in ink in the ink reservoir at one end and coupled to an overflow catchbasin open to atmospheric pressure at the other. The capillary attraction of ink away from the reservoir induces a slightly negative pressure in the reservoir. This underpressure increases as ink is ejected from the reservoir. When the underpressure reaches a threshold value, it draws a small volume of air in through the capillary tube and into the reservoir, thereby preventing the underpressure from exceeding the threshold value.
While the foregoing two techniques for maintaining the ink pressure below ambient have proven highly satisfactory and unique in many respects, they nevertheless have certain drawbacks. The bladder system, for example, is not as volumetrically efficient as might be desired. To minimize the variability of underpressure as a function of reservoir volume, the bladder is desirably of rounded shape. Best volumetric efficiency is obtained, however, if the bladder has a rectangular shape. (Even with a rounded shape, the underpressure is still a function of the bladder's state of collapse and eventually increases to the point that no more ink can be drawn therefrom, even though ink in the reservoir is not exhausted.)
The capillary system suffers with environmental excursions. If the ambient temperature increases, or if the ambient pressure decreases, the air trapped inside the ink reservoir expands. This expansion drives ink from the reservoir and out the printhead nozzles where it may contact the user.
Consequently, it is an object of the present invention to provide an ink jet ink reservoir that overcomes these drawbacks of the prior art.
It is a more particular object of the present invention to extend the pressure and temperature range over which a volumetrically efficient ink jet ink reservoir can operate without leaking.
According to one embodiment of the present invention, an ink jet print head is provided with an ink reservoir having two portions: a fixed volume portion and a variable volume portion. The fixed volume portion can be a rigid chamber. The variable volume portion can be a flexible bladder in a wall of the rigid chamber. Due to volumetric efficiency considerations, the fixed volume portion is desirably larger than the variable volume portion.
Beneath the reservoir is a catchbasin operated at ambient pressure into which ink can be pressure driven from the reservoir through a small coupling orifice. The coupling orifice serves both to convey ink from the reservoir into the catchbasin and to convey fluid (ink or air) from the catchbasin back into the reservoir, depending on the pressure differential. (Due to its occasional role of introducing air into the reservoir, the orifice is sometimes termed a "bubble generator.")
In normal operation, the partial vacuum left in the reservoir when ink is ejected out the print nozzles first causes the flexible bladder portion of the reservoir to collapse. After a certain amount of ink is ejected from the reservoir, the partial vacuum reaches a point at which it draws air into the reservoir from the catchbasin through the small bubble generator orifice. The orifice is sized to begin this bubbling action at a desired underpressure--five inches of water in the illustrated embodiment. Thereafter, as printing continues, the additional underpressure caused by the continued ejection of ink is regulated by the introduction of a corresponding volume of air through the bubble generator orifice.
If the ambient temperature rises, causing the air in the reservoir to expand (or if the ambient pressure diminishes, with similar effect), the bladder starts to restore and expand towards its uncollapsed state so as to contain the additional reservoir volume. In so doing, the bladder continues to exert the bladder restorative force on the ink, maintaining the pressure in the reservoir below ambient to keep the ink in the pen.
In the foregoing case of rising temperature (or decreasing ambient pressure), the bladder restorative force continues to keep the reservoir at a pressure slightly below ambient until the reservoir volume has increased to fully inflate the bladder. At this point, the bladder can no longer serve as a volumetric accumulator and ink is forced to flow through the bubble generator orifice into the catchbasin. (Ink is not driven out through the print nozzle orifii because these orifii are substantially smaller than the bubble generator orifice. Consequently, they require a higher reservoir pressure to drive ink therethrough. This higher pressure is generally never reached because the bubble generator orifice acts to relieve the reservoir pressure before the higher pressure can be attained.)
When the ambient temperature thereafter falls, causing the air pressure in the reservoir to diminish (or when the ambient pressure rises, or when ink is ejected from the reservoir by printing, all with similar effect), ink is drawn from the catchbasin by the pressure differential until it is exhausted. Thereafter, the bladder collapses until the partial vacuum in the reservoir is sufficient to draw air through the orifice from the catchbasin, as described above.
While the foregoing description has focused on a very particular embodiment of an ink jet pen according to the present invention, the invention can more generally be described as including:
a) an ink reservoir;
b) a print head for ejecting ink from the reservoir and thereby leaving a negative pressure therein;
c) a first pressure control mechanism for limiting the negative pressure in the ink reservoir by controllably introducing replacement fluid (i.e. air or ink) thereto; and
d) a second pressure control mechanism for limiting the negative pressure in the ink reservoir by changing the volume thereof.
The foregoing and additional objects, features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.